Cancer remains one of the leading causes of death in the Western world. Clinically, the treatment of human cancer currently involves the use of a broad variety of medical approaches, including surgery, radiation therapy and chemotherapeutic drug therapy (see, for example, the Oxford Textbook of Oncology, Souhami R L, Tannock I, Hohenberger P, and Horiot J-C (ed.s), 2nd edition, New York, N.Y., Oxford University Press, 2002).
A diverse group of chemotherapeutic agents are used in the treatment of human cancer, including the taxanes paclitaxel and docetaxel, the topoisomerase inhibitors etoposide, topotecan and irinotecan, the antimetabolites methotrexate, 5-fluorouracil, 5-fluorodeoxyuridine, 6-mercaptopurine, 6-thioguanine, cytosine arabinoside, 5-aza-cytidine and hydroxyurea; the alkylating agents cyclophosphamide, melphalan, busulfan, CCNU, MeCCNU, BCNU, streptozotocin, chlorambucil, bis-diamminedichloroplatinum, azetidinylbenzoquinone; the plant alkaloids vincristine, vinblastine, vindesine, and VM-26; the antibiotics actinomycin-D, doxorubicin, daunorubicin, mithramycin, mitomycin C and bleomycin; and miscellaneous agents such as dacarbazine, mAMSA and mitoxantrone. However, some neoplastic cells develop resistance to specific chemotherapeutic agents or even to multiple chemotherapeutic agents, and some tumours are intrinsically resistant to certain chemotherapeutic agents. Such drug resistance or multiple drug resistance can theoretically arise from expression of genes that confer resistance to the agent, or from lack of expression of genes that make the cells sensitive to a particular anticancer drug.
It is well established that certain pathological conditions, including cancer, are characterized by the abnormal expression of certain molecules, and these molecules thus serve as “markers” for a particular pathological condition.
Apart from their use as diagnostic “targets”, i.e. abnormal components that can be identified to diagnose the pathological condition, the molecules serve as reagents which can be used to generate diagnostic and/or therapeutic agents. An example of this, which is not intended to be limiting, is the use of markers of cancer to produce antibodies specific to a particular marker. A further non-limiting example is the use of a peptide which complexes with an MHC molecule, to generate cytolytic T cells against cells expressing the marker.
One particular cancer target of interest is colorectal cancer. Colorectal cancers are the third most common malignancies in the world, and amongst men in the European Union it is the second most common cause of cancer death after lung cancer. Although more than 90% of cases are curable when diagnosed at an early stage in development, the majority of patients with colorectal cancer present clinically when the tumour is at an advanced, metastatic stage. Consequently, the disease kills around 98,500 people every year in the EU (where less than 50% of patients survive 5 years after an initial diagnosis of colorectal cancer) and an estimated 437,000 people per annum worldwide. This problem of late diagnosis is compounded by the resistance of some patients' tumours to currently available chemotherapy; leading to a failure to respond to treatment. Such patients require earlier detection and more successful treatment of their illness, and to this end it is desirable to identify proteins whose expression is associated with cancerous cells, which may serve as diagnostic markers, prognostic indicators and therapeutic targets.
Colorectal cancer is a consequence of pathologic transformation of normal cells of the colonic epithelium to an invasive cancer, and may result from inherited mutation, spontaneous mutation or exposure to carcinogens in the bowel contents. The majority of cancers of the colorectum are adenocarcinomas (Jass & Morson, J. Clin. Pathol. 40: 1016-23, 1987), but questions remain concerning the true origins of colorectal carcinomas. Such carcinomas may arise both from within existing benign neoplasms (“adenomas”), in what has been termed the adenoma to carcinoma sequence (Muto et al, Cancer 30: 2251-70, 1975), but the majority of adenomas do not appear to progress to carcinoma and indeed may even regress (Knoemschild, Surg. Forum XIV: 137-8, 1963). Alternatively carcinomas may arise de novo from areas of generalised dysplasia without an adenomatous stage. Clinical evidence supports the identification of environment, diet, age and sex as risk factors for colorectal cancer, but the lack of confirmed involvement of these factors in all cases suggests an underlying genetic basis for colorectal tumour formation. Several genetic alterations have been implicated in development of colorectal cancer, including mutations in tumour-suppressor genes, proto-oncogenes and DNA repair genes (reviewed by Robbins & Itzkowitz, Med Clin North Am 86:1467-95, 2002; Fearnhead et al, Br Med Bull 64: 27-43, 2002). For example, WO 0077252 identifies the Barx2 gene as a candidate tumour suppressor implicated in ovarian and colorectal cancer.
One of the earliest detectable events, which may be the initiating event in colorectal tumourigenesis, is inactivating mutation of both alleles of the adenomatous polyposis coli (APC) tumour suppressor gene. Other implicated genes include MCC, p53, DCC (deleted in colorectal carcinoma), and genes in the TGF-beta signalling pathway. Tumour specific patterns of expression have also been demonstrated for a number of proteins in colorectal tissues, and these proteins are undergoing evaluation as diagnostic and therapeutic targets. One such protein is carcinoembryonic antigen (CEA), which is detectable in the majority of colorectal cancers but not in normal tissues (reviewed by Hammarstrom, Semin Cancer Biol 9: 67-81, 1999). CEA is immunologically detectable in the serum of colorectal cancer patients, and detection of CEA mRNA by RT-PCR can identify lymph node micrometastases, which are a prognostic indicator of a reduced chance of survival in colorectal cancers (Liefers et al, New England J. of Med. 339: 223-8, 1998). Another promising marker for colorectal cancer is minichromosome maintenance protein 2 (MCM2), which is being developed as a target for diagnosis from stool samples (Davies et al, Lancet 359: 1917-9, 2002).
At the present time, none of the protein markers under investigation are in routine clinical use, and further targets for diagnosis, prognosis and treatment are desirable. The current routine diagnostic test for colorectal cancer is the FOBT (Faecal Occult Blood Test), which is lacking in sensitivity and specificity. Evaluation of the effectiveness of this test indicates that it may fail to detect as many as 76% of suspicious growths (Lieberman et al, N Engl J Med; 345: 555-60, 2001). It also results in a large number of false positives and these patients require to undergo the unpleasant, invasive procedure of colonoscopy. Even when administered together these two procedures have been found to miss 24% of tumours and precancerous polyps. Currently the best candidates for new diagnostic tests are based on DNA analysis, but these have at best a 50% detection rate.
It will be appreciated from the forgoing that the provision of novel specific, reliable markers that are differentially expressed in normal and transformed tissues (such as colorectal tissue) would provide a useful contribution to the art. Such markers could be used inter alia in the diagnosis of cancers such as colorectal cancer, the prediction of the onset of cancers such as colorectal cancer, or the treatment of cancers such as colorectal cancer.